I'm posting this in case anyone has experienced the same problems. I have a Winchester SX3 that I love dearly. I bought it used and I don't quite know how many rounds were through it prior to me owning the gun, but it didn't look like much at all. Anyway, so far this season I've put about 500 rounds through it, mostly 3". I recently made the switch to 3.5" and my shells weren't cycling. It would fire and eject, but a new round would not load from the mag tube. I cleaned the gun absolutely thoroughly as I do almost every time I fire it, but I was missing a crucial part. If you are experiencing a similar problem, try cleaning the recoil spring in the stock. I didn't even take the spring out. I sprayed Powder Blast in the tube, let it soak, then followed it up with some RemOil a few minutes later. Now it operates flawlessly again. To avoid this issue in the future, however, I may be switching to a stainless steel recoil spring/tube assembly.

Make sure the chamber is empty and the bolt is closed.Remove the recoil pad, and then remove items 69, 71 and 70. You need a 3/4" socket for the nut (item 69).

Remove the butt stock.

Then, find a dowel or other similar tool about 3/8" in diameter, and use it to push in on item 3. This will take the tension off of the pin (item 4) and allow it to slide out.

The spring is under tension. Once the pin is out, the only thing keeping the spring in the tube is the dowel that you are pushing on. So, when you back the dowel out, be careful not to let the spring fly out and put out an eye.